1. Field of the Invention
The present invention relates to a light receiving circuit.
2. Description of Related Art
In recent years, optical disk drive devices capable of recording/reproducing data using optical disk media such as CDs/DVDs have become widespread. A light receiving IC provided in the pick-up unit of such optical disk drive devices has functions of converting a laser light reflected from a disk into an electrical current by using a light-sensitive element, and of converting the converted electrical current into a voltage by using an I-V conversion amplifier.
Note that the intensity of light input to the light receiving IC changes depending on the operating situation due to the difference in the intensity of laser output according to recording/reproducing operations and the difference in the reflectivity from one optical disk medium to another. Therefore, to cope with recording/reproducing operations of various optical disks, it is necessary to switch the I-V conversion gain according to the intensity of input light. That is, the light receiving IC needs to be equipped with a function of switching an I-V gain. Further, as a gain switching characteristic, such switching needs to be performed in a short time and with stability, for example, for data/address verifications at the time of disk recording.
Such light receiving circuit having an I-V conversion gain switching function is disclosed in Japanese Unexamined Patent Application Publication No. 2007-110021, for example. The technique disclosed in Japanese Unexamined Patent Application Publication No. 2007-110021 is referred to as Publicly-known technique 1 hereinafter. FIG. 5 is a circuit diagram showing a configuration of a light receiving circuit disclosed in Publicly-known technique 1. Note that FIG. 5 shows only a part of a light receiving circuit related to the present invention.
As shown in FIG. 5, two I-V conversion amplifiers 8 and 9 having different I-V conversion gains are provided for one photodiode 6. Power-supply voltage terminals 1, 2 and 3 are connected to the I-V conversion amplifiers 8 and 9. A power-supply voltage Vcc, a ground voltage GND, and a reference voltage Vref are supplied through the power-supply voltage terminals 1, 2 and 3 respectively. Further, a bias terminal 4 is connected to the I-V conversion amplifier 8. Meanwhile, a bias terminal 5 is connected to the I-V conversion amplifier 9. Then, by selectively turning on the bias terminal 4 or 5 corresponding to the I-V conversion amplifier 8 or 9 that has a desired I-V conversion gain, the gain switching between the I-V conversion amplifiers 8 and 9 is implemented. Further, a voltage converted with the desired I-V conversion gain is output from an output terminal 10.
However, the following problem arises in the light receiving circuit of Publicly-known technique 1. To switch the I-V conversion amplifier in a short time and with stability, it is necessary that a transistor(s) constituting the I-V conversion amplifier on the turning-off side should be turned off swiftly without having any effect on the I-V conversion amplifier on the turning-on side. The light receiving circuit of Publicly-known technique 1 includes a typical output stage circuit having a current mirror configuration including transistors that are driven by a non-constant current like the transistors Q1_3 and Q2_3. Therefore, the transient current that is generated at the time of turning off is not processed swiftly, and has an adverse effect on the output characteristics of the I-V conversion amplifier on the turning-on side. As a result, excellent switching characteristics cannot be obtained.
As a specific example, a case where the bias terminal 4 is changed from an Off-state to an On-state and the bias terminal 5 is changed from an On-state to an Off-state in FIG. 5 is explained hereinafter. That is, an operation at the moment at which the I-V conversion amplifier 8 is changed from an Off-state to an On-state and the I-V conversion amplifier 9 is changed from an On-state to an Off-state is explained.
The transistors Q2_2 and Q2_3 form a current mirror configuration. Therefore, when the bias terminal 5 is in an Off-state, the transistor Q2_2 is turned off. As a result, the transistor Q2_3 is also turned off. The transistor Q2_3 needs to be turned off swiftly at the time of switching between the I-V conversion amplifiers 8 and 9. However, since the transistor Q2_4 is turned off at the same time, the transient current from the collector of the transistor Q2_3 is processed as the base current of the transistor Q2_5. Therefore, the transistor Q2_3 cannot be swiftly turned off. Further, since the transistor Q2_6 is in an Off-state, the emitter current of the transistor Q2_5 that is generated by this transient current of the transistor Q2_3 affects the I-V conversion amplifier 8 through the feedback resistor R2_1. Further, depending on the presence/absence of a light input current 7, one of the following phenomena occurs.    (1) Light input current 7 is present
Since the emitter current of the transistor Q2_5 transiently reduces the light input current 7 to the I-V conversion amplifier 8, an abnormal output is observed at the output terminal 10. FIG. 2 shows an output waveform from the output terminal 10 at the time of switching between the I-V conversion amplifiers. As indicated by a dashed-dotted line in FIG. 2, the Vout waveform (output waveform) has a blunted shape.    (2) Light input current 7 is absent
The emitter current of the transistor Q2_5 flows to the transistor Q1_6 of the output stage circuit of the I-V conversion amplifier 8 through the feedback resistor R1_1. That is, a current path is formed from the emitter of the transistor Q2_5 to the transistor Q1_6 through the feedback resistor R1_1, and an abnormal output is thereby observed at the output terminal 10.
It is conceivable that a constant current circuit may be added, for example, as shown in FIG. 6 in order to process the transient current that is generated when the transistors Q1_3 and Q2_3 are turned off as described above. The technique like this is referred to as Publicly-known technique 2 hereinafter. FIG. 6 is a circuit diagram showing a configuration of a light receiving circuit disclosed in Publicly-known technique 2.
FIG. 6 shows a light receiving circuit in which two additional transistors Q1_7 and Q2_7 are added in the light receiving circuit shown in FIG. 5. Further, the collector of the transistor Q1_7 is connected to the collector of the transistor Q2_3, and the base of the transistor Q1_7 is connected to the bias terminal 4. Furthermore, the collector of the transistor Q2_7 is connected to the collector of the transistor Q1_3, and the base of the transistor Q2_7 is connected to the bias terminal 5. In this way, the transient current, which is generated when the transistors Q1_3 and Q2_3 are turned off and which is the origin of the above-mentioned problem, is processed.
As a specific example, a case where the bias terminal 4 is changed from an Off-state to an On-state and the bias terminal 5 is changed from an On-state to an Off-state in FIG. 6 is explained hereinafter. That is, an operation in the case in which the I-V conversion amplifier 8 is changed from an Off-state to an On-state and the I-V conversion amplifier 9 is changed from an On-state to an Off-state is explained. As with the case explained above, the transient current is generated when the transistor Q2_3 is turned off. This transient current can be processed by operating the constant current circuit by the transistor Q1_7 by using the bias terminal 4, which is the bias on the turning-on side. However, another problem described below arises.
After the constant current circuit of the transistor Q1_7 has processed the whole transient current of the transistor Q2_3, it continues to operate with a constant current in a state where no current can be supplied from the collector side. Therefore, it begins the saturation region operation of the transistor in which a current is drawn from the base side. As a result, the collector potential of the transistor Q1_7, i.e., the collector potential of the transistor Q2_3 falls to or below the base potential of the transistor Q1_7. When the I-V conversion amplifier 9 is changed from the Off-state to an On-state in this state, the collector potential of the transistor Q2_3 needs to return to the operating potential from this lowered potential state, and thereby causing an abnormal waveform. A specific output waveform of this behavior is indicated by a dotted line in FIG. 2. As shown in FIG. 2, although the blunted shape of the waveform at the time of switching between the I-V conversion amplifiers in Publicly-known technique 1 is alleviated, a groove-shaped abnormality occurs in the waveform.